Water-in-Ionic Liquid Microemulsion Formation in Solvent Mixture of Aprotic and Protic Imidazolium-Based Ionic Liquids

We report that water-in-ionic liquid microemulsions (MEs) are stably formed in an organic solvent-free system, i.e., a mixture of aprotic (aIL) and protic (pIL) imidazolium-based ionic liquids (ILs) containing the anionic surfactant dioctyl sulfosuccinate sodium salt (AOT). Structural investigations using dynamic light, small-angle X-ray, and small-angle neutron scatterings were performed for MEs formed in mixtures of aprotic 1-octyl-3-methylimidazolium ([C₈mIm⁺]) and protic 1-alkylimidazolium ([C_<i>n</i>ImH⁺], <i>n</i> = 4 or 8) IL with a common anion, bis­(tri­fluoro­methanesulfonyl)­amide ([TFSA^–]). It was found that the ME structure strongly depends on the mixing composition of the aIL/pIL in the medium. The ME size appreciably increases with increasing pIL content in both [C₈mIm⁺]­[TFSA^–]/[C₈ImH⁺]­[TFSA^–] and [C₈mIm⁺]­[TFSA^–]/[C₄ImH⁺]­[TFSA^–] mixtures. The size is larger for the <i>n</i> = 8 system than that for the <i>n</i> = 4 system. These results indicate that the shell part of MEs is composed of both AOT and pIL cation, and the ME size can be tuned by pIL content in the aIL/pIL mixtures